Performance of Light Emitting Diodes (LEDs) has increased significantly over the last ten years following Haitz's Law who has predicted that LED brightness doubles every 18 to 24 months. LEDs with power efficiency exceeding incandescent lamps have been developed and are available in the market. Unlike fluorescent lights which contain mercury, LEDs are believed to be more environmentally friendly. LEDs may be used in both low power areas such as indicator lights for electronic devices and also high power applications such as flash lights, backlighting displays, electronic signs and signal displays. Today, many strobe lights found in cameras and flashlights use LEDs.
Although LEDs have many advantages, there are challenges in terms of designs and mass volume manufacturing. One drawback is that light efficiency and life span of LEDs depend on operating temperature. Therefore, heat dissipation becomes a consideration in LED package designs. A good LED package design will enable heat dissipation, which lowers junction temperature and increases efficiency. Another consideration is the viewing angle. Light generated by LEDs may not necessary be emitted at the desired angle. A reflector is needed to ensure that light emitted from an LED die is directed to a desired direction or angle. Another drawback of LEDs is process variation. Sophisticated controls steps in manufacturing at the expense of cost may be needed to reduce process variation. The discussions above are general considerations in LED designs. Depending on the application, certain aspects can be ignored whereas certain aspects will become more critical.
Although optic design is mainly done at the package level, work on additional secondary optics is still needed at the application level in certain situations. Examples of the secondary optics are additional external reflectors and lens that can be placed onto LEDs. When packaged LEDs do not meet the specific requirement at the application level, optic designers use additional reflectors to improve light mixing. Additional reflectors optimize the light extraction to a desired viewing angle. For example, a flashlight may only require the light to illuminate at certain angle but a packaged LED may produce light at wider angle. This gap is filled by the additional second level optics designs such as reflector cups.